1. Field of the Invention
This invention relates to a refractive optical configuration for a plural field-of-view forward looking infrared (FLIR) sensor.
2. Brief Description of the Prior Art
Airborne FLIR sensors are generally housed in a truncated spherical envelope typical of that used in a two-axis mass gimbal configuration. The truncation is necessary to accommodate the gimbal structure secured thereto. The size of the spherical housing is usually determined by the minimum diameter required to encompass the optical configuration. As a result, the optics utilize the majority of the volume within the truncated sphere and little room is left for other necessary components, such as the detector and its cooling apparatus and the electronics. Space for other electro-optical devices is therefore also very restricted.
Furthermore, the maximum size of the prior art FLIR aperture diameter has been limited to, at most, less than 45 percent of the diameter of the spherical housing due to the size of the optics and the loss in volume within the housing for a flat window to cover the optical aperture and the sag of the window within the spherical envelope. These flat windows also cause drag variations on the gimbal as a function of gimbal angle, making stabilization difficult for high resolution.
Existing FLIR sensors are designed for a given number of detector elements, such as 240 or 480, and require a total redesign of the optics and sensor to change these performance characteristics.
All known prior art FLIR systems have enclosed the FLIR optical components within the minimum size spherical envelope as discussed above or increased the size of the optics to fill a known spherical size. The other components are fitted into the remaining space which is usually minimal. In the case of an aircraft pod, such as that on the F18 aircraft, a roll over pitch gimbal is employed. The truncated spherical envelope contains only optical components and additional optics are required to relay the image to the detector and other components located on other parts of the gimbal.
In summary, in the prior art, the optical aperture in any known gimbal FLIR does not exceed 45 percent of the spherical envelope diameter and the optics and the window extend from the front to the rear of the sphere. All known prior art high resolution FLIRs use flat windows in the spherical housing. If the FLIR aperture is large compared to the diameter of the spherical housing, the sag of the flat window from the spherical shape is also large. This makes the aerodynamic drag vary as a function of gimbal angle and causes large torque variations on the gimbal. No known prior art FLIRs have the capability to change detector arrays from 240 element to 480 element.